Positive flow control systems are known that use closed-centered valves connected in parallel in combination with a separate by-pass valve to control the flow of fluid in the system in response to the operator's input command. These known positive flow control systems do not permit priority control without providing special controls. Furthermore, an extra bypass valve is required to control the flow from the pump whenever the control valves are in their closed positions. Even though the pumps may be flow/pressure compensated, it is desirable to maintain some minimum flow and pressure in order to have a supply of pressurized fluid for the system's control, for example; pilot control fluid, lubrication fluid or make-up fluid. It is also known, to place open-centered valves in an interrupted series relationship in order to provide priority flow to a desired function. For example, it may be desirable to ensure that the tilt actuator of a wheel loader have priority over the lift actuator. In these known open-centered systems, whenever the control valves are in their centered positions, all of the flow from the pump is passed therethrough to the reservoir. In earlier systems the pump may have been at maximum displacement and all of the energy needed to produce the flow is wasted. In subsequent systems, a flow restriction has been placed in the tank line downstream of the last control valve in the series in order to control the rate of flow to the reservoir. In order to offset the high increase in system pressure due to the addition of the restriction, the pressure drop across the restriction was sensed and used to control the displacement of the pump. This type of system has normally been referred to as negative flow control. Negative flow control systems rely on the exhausted flow in the system to control the flow rate from the pump. These types of systems still permit a large amount of fluid flow to return to the reservoir as the open-centered control valve is being moved to an actuated position.
It is desirable to minimize the volume of fluid flow that is permitted to be passed to the reservoir in order to minimize horsepower losses. It is also desirable to minimize flow losses while also providing priority to selected functions without the need for complicated controls.
The present invention is directed to overcoming one or more of the problems as set forth above.